Crystal structure of a methyl benzoate quadruple-bonded dimolybdenum complex

The quadruple-bond complex, [Mo2(p-O2CC6H4CH3)4·2(C4H8O)]·2C4H8O, crystallizes within a triclinic space group P . The slightly electron donating group on the paddlewheel carboxylate together with the axial THF negligibly perturbs the Mo—Mo bond distance.


Chemical context
The two-electron bond (Lewis, 1916) is the most basic element in the field of chemistry. Quadruple-bond complexes have been central in experimentally defining the two-electron bond within a unified context of the valence (Heitler & London, 1927) and molecular orbital (Pauling, 1928;Lennard-Jones, 1929;Mulliken, 1932;James & Coolidge, 1933;Coulson & Fischer, 1949) bonding models. The importance of quadruplebond complexes in elucidating the two-electron bond arises from the four states that originate from the two-orbital electron configuration: 1 '', 3 ''*, 1 ''*, and 1 '*'*, where ' and '* represent bonding and antibonding orbitals, respectively. In experimental systems with and bonding frameworks, the excited states are not all accessible because of the dissociation or rotation arising from population of and antibonding orbitals. Quadruple-bonded metal-metal complexes, whose metal-metal linkages are characterized by a 2 4 2 ground state, are able to overcome this limitation. Pioneered by a 2 4 framework and locked from rotation by diametrically opposed bulky ligands or bidentate ligands, all four states defining the 2 two-electron bond ( 1 , 3 *, 1 *, and 1 **) may experimentally be verified for dimolybdenum quadruple-bond complexes (Engebretson et al., 1994(Engebretson et al., , 1999Cotton & Nocera, 2000;Boettcher et al., 2022).
In the preliminary investigation of Mo 2 (O 2 CCH 3 ) 4 , Lawton & Mason (1965) determined the dimolybdenum bond distance to be 2.11 Å , which was later adjusted by Cotton & Norman (1971) to 2.0934 Å . As a result of the weak overlap of the d xy orbitals constituting a bond, one-electron oxidation or reduction of a dimolybdenum core does little to perturb the dimolybdenum bond distance, allowing for the spectro-electrochemical determination of the degree of overlap between these orbitals (Boettcher et al., 2022).
How the properties of the equatorial ligands affect the dimolybdenum bond distance has been a central question in the structural chemistry of quadruple-bond complexes (Han, 2011). Cotton proposed that either electron-withdrawing or electron-donating substituents in the ligand field of the dimolybdenum core will modulate the bonding within the quadruple-bond framework (Cotton et al., 1978). A comparative analysis of electron-donating, -neutral and -withdrawing ligands drives to the heart of this issue. Previous studies have examined the electron-neutral Mo 2 (p-O 2 CC 6 H 5 ) 4 (Cotton et al., 1978)  The presence of an electron-donating methyl group on the bridging benzoate ligands results in a minor elongation of the dimolybdenum bond with respect to the parent benzoate compound and compression in comparison to a benzoate complex with an electron-withdrawing trifluoromethyl group.

Structural commentary
The molecular structure of the dimolybdenum complex, [Mo 2 (p-O 2 CC 6 H 4 CH 3 ) 4 Á2(C 4 H 8 O)] is presented in Fig. 1 as ascertained using single-crystal X-ray diffraction. The asymmetric unit contains half of the molecule (Fig. 1) (Cotton et al., 2002). Noting that the dimolybdenum bond distance of the unsubstituted phenyl analogue, Mo 2 (O 2 CC 6 H 5 ) 4 , is 2.096 (1) Å (Cotton et al., 1978), the addition of the methyl group at the 4-position of the benzoate results in an increase of the dimolybdenum bond distance by 0.0053 (1) Å .

Supramolecular features
Molecular packing arrangements are shown in Fig. 2. The structure was solved in the triclinic space group P1. Unbound THF molecules are ordered in between p-O 2 CC 6 H 4 CH 3 ligands of adjacent molecules, along the b-axis, with the oxygen atom facing away from the metal center and toward the methyl groups. The O2 oxygen atoms of the unbound THF solvent molecules are located at distances of 4.178 (3) and 6.530 (4) Å from the C13 atoms of the p-O 2 CC 6 H 4 CH 3 ligands of adjacent molecules.

Database survey
A search in the Cambridge Structural Database (WebCSD, accessed November 2022; Groom et al., 2016) (Cotton et al., 1978) is MOBZOA.   THF, and 1,2-dichlorobenzene were purchased from Sigma-Aldrich. Mo(CO) 6 and p-toluic acid were combined in an oven-dried flask with anhydrous THF and anhydrous 1,2-dichlorobenzene. The reaction was heated under reflux for 48 h at 413 K under a dry N 2 atmosphere (Pence et al., 1999). The reaction mixture was cooled, dried, and washed with anhydrous dichloromethane and pentane.

Synthesis and crystallization
The crystallization was prepared in a glove box. The crude product was dissolved in anhydrous THF, filtered, and recrystallized by vapor diffusion of pentane using a 6 by 50 mm borosilicate glass crystallization tube housed within a 20 mL glass vial. The assembly was allowed to stand at 238 K for 14 days. Orange block-shaped crystals were observed and harvested for X-ray diffraction analysis. Table 2 contains crystal data, data collection, and structure refinement details. A single orange block (0.220 mm Â 0.180 mm Â 0.140 mm) was chosen for single-crystal X-ray diffraction using a Bruker D8 goniometer equipped with an Photon100 CMOS detector. Data were collected as a series of ' and/or ! scans. Data integration down to 0.84 Å resolution was carried out using SAINT V8.37A with reflection spot size optimization. Absorption corrections were made with the program SADABS2016/2 (Krause et al., 2015). Space-group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the struc-tures. The structure was solved by the intrinsic phasing method and refined by least-squares methods also using SHELXT2014/5 and SHELXL2014/7 with the OLEX2 (Dolomanov et al., 2009) interface. The program PLATON (Spek, 2020) was employed to confirm the absence of higher symmetry space groups. All non-H atoms, including the disorder fragment, were located in difference-Fourier maps, and then refined anisotropically. Outlier reflections were omitted from refinement when appropriate. Hydrogen atoms on C atoms were placed at idealized positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the atoms they are linked to (1.5 times for methyl groups). Crystallographic refinement details, including the software employed, have been delineated within the crystallographic information (*.cif).

Refinement
Acta Cryst. (2023). E79, 231-235 research communications Computer programs: SAINT (Bruker, 2015), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2014/7 (Sheldrick, 2015b) and SHELXTL (Sheldrick, 2008). (Sheldrick, 2008). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on all data will be even larger.